Roughness – the presence of irregularities on the surface of a material – is commonly thought to be a source of stickiness and slower motion. Molecules feel differently about such a situation.
At ambient scale, it takes longer to walk up and down a hilly path than a flat one. Roughness also taxes smaller objects. Italians make coarse pasta – rigatoni – to have tomato sauce and cheese better adhere.
The rules change at the nanometer (billionth of a meter) scale. Molecules of different sorts have individual preferences for personal space and arrangement in space.
Most molecular clusters tend to be spherical, with shells of atoms around a central atom. Gold is exceptional, in forming freestanding clusters in a pyramidal shape.
To the surprise of physical chemists, molecules near a rough substrate behave as if they were surrounded by fewer neighbors. Feeling liberated, they move with ease. As simply put by American chemist Jane Lipson: “the faster interfacial dynamics originate from a reduction in interfacial density, due to the noncomplete filling of substrate asperities.”
With their tender bums, polymers do not enjoy sitting on a rough substrate. They prefer to stray away from asperities. With room to roam, the few remaining molecules can more quickly bust a move.
Anna Panagopoulou et al, “Substrate roughness speeds up segmental dynamics of thin polymer films,” Physical Review Letters (15 January 2020).
“Molecules move faster over rough terrain,” ScienceDaily (17 January 2020).
Zhe Li et al, “Unraveling the atomic structure, ripening behavior, and electronic structure of supported Au20 cluster,” Science (3 January 2020).
“Clusters of gold atoms form peculiar pyramidal shape,” ScienceDaily (3 January 2020).